1. Field of the Invention
The present invention relates to an apparatus and method of obtaining an intensity distribution of radiation transmitted through an object, i.e., a radiation image of an object.
2. Description of the Related Art
Image sensing methods of irradiating an object with radiation and detecting the intensity distribution of radiation transmitted through the object to obtain a radiation image of the object are generally extensively used in industrial nondestructive inspection and medical diagnoses. Of practical image sensing methods of obtaining a radiation image of an object, the most general approach is to combine a so-called xe2x80x9cfluorescent screenxe2x80x9d (or intensifying screen) which emits fluorescence when irradiated with radiation with a silver halide film. In this method, the fluorescent screen is irradiated with radiation via an object. The fluorescent screen converts the radiation into visible light and the light forms a latent image on the silver halide film. This silver halide film is chemically treated to obtain a visible image. Radiation images obtained by this image sensing method are so-called analog photographs and used in medical diagnoses, nondestructive inspection, and the like.
Also, a computed radiography apparatus (to be referred to as a CR apparatus hereinafter) using an imaging plate (to be referred to as an IP hereinafter) coated with a stimulative phosphor is beginning to be used. When the IP primarily excited by radiation is secondarily excited by light such as an infrared laser beam, the IP emits light called stimulative fluorescence. The CR apparatus obtains image data of a radiation image by detecting this stimulative fluorescence by using a photosensor such as a photomultiplier. On the basis of this image data, the CR apparatus forms a visible light image on, e.g., a photosensitive material or a CRT. Although this CR apparatus is a digital apparatus, the CR apparatus is an indirect digital radiation image sensing apparatus because an image formation process, i.e., reading by secondary excitation is necessary. The reason why the apparatus is called xe2x80x9cindirectxe2x80x9d is that it cannot instantly display sensed radiation images like in analog technologies.
On the other hand, a technology is recently developed by which digital images are obtained by using a photoelectric converter in which light-receiving elements such as fine photoelectric converting elements and switching elements are arranged in a matrix manner. Radiation image sensing apparatuses in which a phosphor is stacked on an CCD or an amorphous silicon two-dimensional image sensing element are disclosed in U.S. Pat. Nos. 5,418,377, 5,396,072, 5,381,014, 5,132,539, and 4,810,881. These radiation image sensing apparatuses can instantly display obtained radiation images and hence can be called direct digital radiation image sensing apparatuses.
The digital radiation image sensing apparatuses have the following advantages over the analog photographing technologies. That is, no film is necessary, and obtained information can be enlarged by image processing and used in the form of a database. Also, the advantage that the direct digital image sensing apparatuses have over the indirect digital radiation image sensing apparatuses is instantaneity. The ability to display sensed radiation images on the spot is particularly useful on urgent medical scenes.
Unfortunately, a two-dimensional solid-state image sensor has a dark current which is one cause of noise, so the image sensing time of the radiation image sensing apparatus cannot be indiscriminately increased. Therefore, a radiation image is obtained by exchanging signals with a radiation generator and synchronizing the irradiation timing with the image sensing timing of the radiation image sensing apparatus following one of image sensing procedures below.
(1) Upon receiving an image sensing request signal from the radiation generator, the radiation image sensing apparatus initializes the apparatus. When the initialization is complete, the radiation image sensing apparatus sends an image sensing preparation completion signal to the radiation generator. When a preset irradiation time has elapsed, the radiation generator terminates the irradiation and sends an irradiation end signal co the radiation image sensing apparatus. Upon receiving the irradiation end signal, the radiation image sensing apparatus terminates accumulation of output from the photoelectric converting elements and switches the operation mode to an image data output operation.
(2) If no irradiation end signal can be obtained because of the type of interface, the radiation image sensing apparatus measures the time. When almost the same time as the irradiation time set in the radiation generator has elapsed, the radiation image sensing apparatus terminates accumulation of the outputs from photoelectric converting elements and starts the image data output operation.
In image sensing procedure (1) in which the radiation generator controls the radiation image sensing apparatus, the interface between the two apparatuses is complicated. Also, the radiation generator and the radiation image sensing apparatus are enlarged and complicated because these apparatuses must be constructed as one system. This makes this radiation image sensing apparatus difficult to be used as an on-vehicle radiation image sensing apparatus used in group examination or a portable radiation image sensing apparatus for sensing radiation images on the bed side.
In image sensing procedure (2), the radiation image sensing apparatus cannot check the end of irradiation, so it cannot be ensured that the irradiation is really complete. The radiation image sensing apparatus can also be previously given an accumulation time for accumulating output from the photoelectric converting elements so as to be capable of compensating for variations in the irradiation time in the radiation generator. However, it is still necessary to measure the individual characteristics of the radiation generator and calibrate the radiation image sensing apparatus when these apparatuses are installed. Additionally, since unnecessary output from the photoelectric converting elements is accumulated, extra noise components are added to obtained image data by a dark current. Furthermore, the irradiation time must be previously acquired from the radiation generator or a control terminal of the radiation image sensing apparatus. This complicates the interface with the radiation generator or makes the operation cumbersome.
It is an object of the present invention to provide a radiation image sensing apparatus and method capable of sensing a radiation image with a simple interface.
It is another object of the present invention to provide a radiation image sensing apparatus and method which obviate the need to measure the individual characteristics of a radiation generator and calibrate the radiation image sensing apparatus when these apparatuses are installed.
To achieve the above objects, one preferred embodiment of the present invention discloses a radiation image sensing apparatus for irradiating an object with radiation to obtain a radiation image, comprising a radiation image sensing unit having a plurality of photoelectric converting elements two-dimensionally arranged in a matrix manner; a sensor for detecting irradiation of radiation; determining means for determining an irradiation state of radiation on the basis of an output from the sensor; and control means for controlling a state of the radiation image sensing unit on the basis of the determination by the determining means.
Another preferred embodiment of the present invention discloses a radiation image sensing apparatus for irradiating an object with radiation to obtain a radiation image, comprising: radiation control means for controlling irradiation of radiation; a radiation image sensing unit having a plurality of photoelectric converting elements two-dimensionally arranged in a matrix manner; a sensor for detecting irradiation of radiation; determining means for determining an irradiation state of radiation on the basis of an output from the sensor; and image sensing control means for setting a state of the radiation image sensing unit, wherein upon receiving a signal indicating a start of image sensing, the radiation control means outputs an image sensing request signal; the image sensing control means having received the image sensing request signal sets the radiation image sensing unit in an image sensing preparation state, and outputs an image sensing preparation completion signal; the radiation control means starts irradiation of radiation when receiving the image sensing preparation completion signal, and terminates the irradiation of radiation when an accumulated irradiation amount reaches a predetermined value or a preset time elapses; the determining means outputs an irradiation start signal indicating a start of irradiation of radiation when an output from the sensor exceeds a predetermined level, and outputs an irradiation end signal indicating an end of the irradiation of radiation when the output from the sensor becomes smaller than a predetermined level; and the image sensing control means switches the radiation image sensing unit from the image sensing preparation state to an image sensing state when receiving the irradiation start signal, and switches the radiation image sensing unit from the image sensing state to an image sensing result output enable state when receiving the irradiation end signal.
Still another preferred embodiment of the present invention discloses a radiation image sensing apparatus for irradiating an object with radiation to obtain a radiation image, comprising: radiation control means for controlling irradiation of radiation; a radiation image sensing unit having a plurality of photoelectric converting elements two-dimensionally arranged in a matrix manner; a sensor for detecting irradiation of radiation; determining means for determining an irradiation state of radiation on the basis of an output from the sensor; and image sensing control means for setting a state of the radiation image sensing unit, wherein the radiation image sensing unit is switched to an image sensing preparation state when a power supply of the apparatus is turned on or an image sensing result is output; radiation is irradiated, when a signal indicating image sensing start is input, until an accumulated irradiation amount reaches a predetermined value or a preset time elapses; the radiation image sensing unit is switched from the image sensing preparation state to an image sensing state when an output from the sensor exceeds a predetermined level; and the radiation image sensing unit is switched from the image sensing state to an image sensing result output enable state when the output from the sensor becomes smaller than a predetermined level.
It is still another object of the present invention to provide a radiation image sensing apparatus and method which reduce the influence of noise produced by a dark current in a photoelectric converting element.
To achieve the above-object, still another preferred embodiment of the present invention discloses a radiation image sensing apparatus for irradiating an object with radiation to obtain a radiation image, comprising: a radiation image sensing unit having a plurality of photoelectric converting elements two-dimensionally arranged in a matrix manner; a sensor for detecting irradiation of radiation; determining means for determining an irradiation state of radiation on the basis of an output from the sensor; and control means for controlling a state of the radiation image sensing unit on the basis of the determination by the determining means wherein the image sensing preparation state comprises refreshing of the photoelectric converting elements and a signal output state which reduces an influence of a dark current flowing through the photoelectric converting elements on an image sensing result.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.